This invention relates to x-ray intensifying screens and to a process for the production of pigments useful therein. These screens, which contain a luminescent material which fluoresces on exposure to x-rays, are used in conjunction with a film sensitive in the selected fluorescence range, especially for medical x-ray investigations.
A large number of luminescent materials have been used for this purpose, but calcium tungstate is used primarily. Since x-rays in excessive dosage can negatively affect the health of the persons investigated, there is, in the medical field a continuing need for intensifying screens with higher degrees of intensification to permit use of minimal x-ray dosages during x-ray investigations.
New luminescent substances developed for x-ray intensifying screens, e.g., gadolinium and yttrium oxide sulfides activated with other rare earths or lanthanum oxide halides activated by rare earths, are very expensive.
A further requirement of medical x-rays is that x-ray intensifying screens have minimal afterglow in order to prevent a weak image of the previous picture being transmitted to the following picture during a very rapid picture sequence (cinematography).
All known luminescent materials for x-ray intensifying screens frequently exhibit an afterglow parallel with intensification. Thus, there is a need for x-ray fluorescent luminescent materials for x-ray intensifying screens which
(1) HAVE A VERY HIGH INTENSIFICATION FACTOR,
(2) EXHIBIT MINIMUM AFTERGLOW,
(3) ARE SIMPLE TO PRODUCE AND
(4) ARE SO STABLE THAT REPRODUCIBLE X-RAY PICTURES CAN BE MADE EVEN AFTER A COMPARATIVELY LONG PERIOD OF REPEATED EXPOSURE TO X-RAYS.
It is known that cadmium tungstate is excited to fluoresce by x-rays. In Chemische Berichte, Vol. 62, page 763 (1929), is reported strong x-ray excitability. However, the yellow-green light emitted was not considered suitable for intensification purposes. Consequently, although cadmium tungstate has been used in fluoroscopic screens for direct visual observation, it has long since been replaced by zinc sulfide, cadmium sulfide and zinc silicate and never found acceptance in the production of x-ray intensifying screens.
The requirements of a luminescent material for x-ray screens are completely different from those for a luminescent material for x-ray intensifying screens. In both cases, a good fluoroescent yield is required but the afterglow has an insignificant effect in the case of x-ray screens. Consequently, luminescent materials used for x-ray screens, such as zinc sulfide, cadmium sulfide and zinc silicate, are completely unsatisfactory for use in x-ray intensifying screens.
Consequently and surprisingly, in view of the prejudice in the literature against cadmium tungstate as luminescent material for intensifying screens, it was not foreseeable that the use of cadmium tungstate, according to the invention, in x-ray intensifying screens provides much better results than luminescent materials previously employed.
Therefore, an object of the invention is the production of intensifying screens which contain cadmium tungstate as x-ray fluorescent luminescent material and to the screens thus-produced.